The present invention concerns a discrete electronic component of the inductive type and a method for manufacturing such components. In particular, these components are used in surface mounting techniques (SMD), particularly inductance coils or transformers.
Manufacturing electronic components for surface mounting is well known, particularly for making resistors or capacitors, but this poses problems for the series manufacture of inductance coils or transformers of millimetric dimensions, because they are currently made separately from each other.
In many electronic applications, electronic components of the inductive type are needed as an interface, for example, between voltage levels provided by a power source and integrated circuit input voltages. These inductive elements are used in particular to even ripples on signals. Often the inductance values need to be high, of the order of mH. Usually, the manufacture of such inductive elements does not pose any problem if ferrite cores are used with electric windings of dimensions of the order of one centimeter. However, when the size of the components has to be reduced, there are serious constraints on the technology to be used to make them with high inductance values.
Likewise, for the manufacture of antennae of small dimensions formed of a winding and a magnetic core, the market needs a technology which allows inexpensive manufacturing of large quantities.
SMD type coils proposed by Coilcraft in Cary, Ill., United States are known, i.e. coils able to be mounted on metal pads made on hybrid structures particularly made of ceramic material. These coils are formed of a magnetic core on which a metal wire is wound around the central part and the ends of which are each connected on a metal pad of end parts on either side of the central part. The metal pads may act as a contact with the corresponding metal pads made on a hybrid structure including connection paths with different electronic components. The value of these coils is at the most 10 μH for dimensions of 3 mm×3 mm×2.5 mm. It is clear that they are made one after the other because it is necessary to wind the wire around each magnetic circuit independently, which requires manufacturing time and a high cost.
U.S. Pat. No. 5,463,365 discloses a coil which includes a magnetic core and a winding part formed of a plurality of laminated sheets including windings arranged in a spiral around the core so as to be coaxial. The connection between the windings located on superposed sheets occurs via metallised holes which are well known to those skilled in the art. This method allows a certain number of sheets or layers to be stacked, particularly sheets made of polyimide resin, depending on the number of turns of metal wires desired for the design of the coil.
The manufacture of the coils specified in this American patent is complicated since, to obtain a component of the SMD type able to be mounted on a hybrid structure, in addition to the arrangement of a magnetic core with its winding stack, the embodiments given have an entire infrastructure with a cover for the two sides of the magnetic circuit and several output terminals not all of which are used if the components only has one winding. The shape of said component may be similar to that of a component with a plastic encapsulation case, which is not suitable for very small dimensions. Moreover, the assembly of this component is effected individually.
U.S. Pat. No. 5,760,671 discloses a transformer having two magnetic flux paths defined by a ferrite magnetic circuit in the shape of an eight, this transformer including a plate formed of stacked layers with printed circuits defining the primary and secondary windings of the transformer. The plate has an opening for the central arm of the magnetic circuit which is surrounded by the windings. These windings are raised from the base of the magnetic circuit by steps arranged in corners of the two openings defined by the magnetic circuit.
This transformer is used for voltages of up to 400 V for dimensions exceeding one centimeter. For these dimensions, the manufacture of such components does not pose any particular problem but it cannot be used as a component of the SMD type. Assembly of the plate with the magnetic circuit in two parts is effected individually, as is the bonding of the two parts of the magnetic circuit.